1. Field of the Invention
This invention relates to a system, and a process, for forming deposited films. More particularly, it relates to a deposited film forming system and a deposited film forming process which are able to form a large-area and uniform plasma and also may cause no difficulty in the power feeding course even when discharged for a long time. In particular, the deposited film forming system and process of the present invention is preferably used to continuously produce semiconductor thin films which constitute photovoltaic devices such as solar cells making use of amorphous silicon or amorphous alloys.
2. Related Background Art
As conventionally known in the art, amorphous silicon can form large-area semiconductor devices relatively with ease compared with crystalline silicon and polycrystalline silicon, because large-area semiconductor films can be produced by plasma CVD (chemical vapor deposition). Accordingly, amorphous silicon films are widely used in semiconductor devices required to have a large area as exemplified by photovoltaic devices for solar cells, photosensitive drums for copying machines, image sensors for facsimile machines and thin-film transistors for liquid-crystal display devices.
These devices have a larger area for each device than devices formed of crystalline semiconductors such as LSIs and CCDs. For example, in the case of solar cells, when having a conversion efficiency of 10%, they must have an area of as large as about 30 square meters per home in order to obtain an output of about 3 kW for supplying electric power consumed in ordinary homes. Thus, each photovoltaic device constituting the solar cells is required to have a reasonably large area.
In the case where the amorphous silicon films are formed, plasma CVD is widely used, which is a process comprising decomposing into plasma a material gas containing, e.g., SiH4 or Si2H6, and forming an amorphous silicon film on a substrate provided in the plasma.
As a system for continuously producing amorphous silicon semiconductor devices by such plasma CVD, a continuous plasma CVD system employing a roll-to-roll arrangement is widely known, which is disclosed in, e.g., U.S. Pat. No. 4,400,409.
This system has a plurality of glow discharge chambers in a row and each glow discharge chamber has a through course, where a sufficiently long beltlike substrate having a desired width is disposed through the inside of each glow discharge chamber and along the course in which it passes sequentially through the insides of the glow discharge chambers, and the beltlike substrate is continuously transported in the lengthwise direction while semiconductor films having the desired conductivity types are formed on the substrate in the glow discharge chambers, thus various semiconductor films can be formed in layers in the specific regions of the substrate. As a result, large-area devices having semiconductor junction can be formed continuously.
Use of such a roll-to-roll type continuous plasma CVD system enables long-time continuous production of devices without stopping the production system.
Now, in the roll-to-roll type system constructed as described above or in a system for forming deposited films on the surface of a substrate which is not beltlike, the present inventors attempted to change frequencies so that discharge can be caused to take place in higher frequency regions. As a result, they have become aware that the following difficulty occurs.
That is, large-area planar electrodes commonly used in a frequency region of about 13.56 MHz make it difficult to form a large-area and uniform plasma because the impedances can not be adjusted or can be matched with difficulty for causing discharge to take place in a higher frequency region. Such a technical problem has been found.
In order to solve this problem, a method is devised in which an antenna-shaped discharge electrode is used in place of the planar electrode to cause plasma to take place. This method brings about advantages that electric power concentrates to the antenna-shaped discharge electrode to readily cause discharge to take place and also improve deposition rate. It, however, has been found that a difficulty occurs on the other hand, such that, when discharged for a long time by this method, the power feeding course through which an electric power is fed to the antenna-shaped discharge electrode comes to have a high temperature to cause a great applied-power loss. As a result, when discharged continuously for a longer time, the discharge may become non-uniform to cause changes with time in the film thickness and film properties of deposited films formed, bringing about another problem that defects may occur to lower the yield of products.
There is a conventional apparatus in which the antenna portion extending in the discharge space is cooled. Since, however, the whole power feeding course can not be cooled by only this means, sufficient temperature control has not been achieved.
The present inventors made extensive studies on the cause of these problems. As a result, they have considered that the problems are due to a dielectric loss caused by applied power when an electric power is applied to the film forming space of the plasma CVD system, or due to the fact that the power feeding course leading to the antenna-shaped discharge electrode comes to have a high temperature as a result of the latter""s exposure to the plasma in the film forming space. Here, where a dielectric material so disposed as to surround the power feeding course is integrally provided, its power supply conductor conducts the heat in a large quantity from the discharge electrode and inevitably heats a power source section connected to the power supply conductor and even a container through the wall of which the power supply conductor is provided. Thus, on the basis of this consideration, the present inventors have solved the above problems and have accomplished the present invention as described below.
An object of the present invention is to provide a deposited film forming system and a deposited film forming process which are able to make applied-power loss occur less, to keep the power feeding course from its temperature rise, to make discharge readily take place even after use for a long time and also to maintain uniform discharge for a long time. Stated more specifically, an object of the present invention is to provide a system, and a process, by which deposited films having a high quality and a superior uniformity can be formed over a large area on a beltlike substrate that moves continuously.
To achieve the above object, the present invention provides a deposited film forming system having at least a vacuum vessel, means for feeding a film-forming material gas into the vacuum vessel, a discharge electrode provided inside the vacuum vessel, used to make the material gas into a plasma, and a power supply conductor for applying a high-frequency power to the discharge electrode; the system comprising:
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel; and
a plurality of dielectric materials at least part of which is disposed between the power supply conductor and the earth shield.
The present invention also provides a deposited film forming process comprising the steps of:
feeding a film-forming material gas into a vacuum vessel provided with a discharge electrode inside the vessel; and
applying a high-frequency power through a power supply conductor for applying the high-frequency power to the discharge electrode to make the material gas into a plasma, to form a deposited film on a substrate provided inside the vacuum vessel;
the high-frequency power being fed to the discharge electrode through the power supply conductor, by the aid of;
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel; and
a plurality of dielectric materials at least part of which is disposed between the power supply conductor and the earth shield.
The present invention still also provides a deposited film forming system having at least a vacuum vessel, means for feeding a film-forming material gas into the vacuum vessel, a discharge electrode provided inside the vacuum vessel, used to make the material gas into a plasma, and a power supply conductor for applying a high-frequency power to the discharge electrode; the system comprising:
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel;
a plurality of dielectric materials at least part of which is disposed between the power supply conductor and the earth shield; and
cooling means for cooling the dielectric material.
The present invention further provides a deposited film forming process comprising the steps of:
feeding a film-forming material gas into a vacuum vessel provided with a discharge electrode inside the vessel; and
applying a high-frequency power through a power supply conductor for applying the high-frequency power to the discharge electrode to make the material gas into a plasma, to form a deposited film on a substrate provided inside the vacuum vessel;
the high-frequency power being fed to the discharge electrode through the power supply conductor, by the aid of;
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel;
a plurality of dielectric materials at least part of which is disposed between the power supply conductor and the earth shield; and
cooling means for cooling the dielectric material;
the high-frequency power being fed while cooling the dielectric material by the cooling means.
The present invention still further provides a deposited film forming system having at least a vacuum vessel, means for feeding a film-forming material gas into the vacuum vessel, a discharge electrode provided inside the vacuum vessel, used to make the material gas into a plasma, and a power supply conductor for applying a high-frequency power to the discharge electrode; the system comprising:
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel;
a dielectric material disposed between the power supply conductor and the earth shield; and
cooling means for cooling the dielectric material.
The present invention further provides a deposited film forming process comprising the steps of:
feeding a film-forming material gas into a vacuum vessel provided with a discharge electrode inside the vessel; and
applying a high-frequency power through a power supply conductor for applying the high-frequency power to the discharge electrode to make the material gas into a plasma, to form a deposited film on a substrate provided inside the vacuum vessel;
the high-frequency power being fed to the discharge electrode through the power supply conductor, by the aid of;
an earth shield so disposed as to surround the power supply conductor inside the vacuum vessel;
a dielectric material disposed between the power supply conductor and the earth shield; and
cooling means for cooling the dielectric material;
the high-frequency power being fed while cooling the dielectric material by the cooling means.
In the deposited film forming system of the present invention, the dielectric material at least part of which surrounds the power supply conductor is provided in plurality, and hence the heat generated in the discharge space can be shut out and the power supply conductor can be prevented from being heated, so that the power supply conductor can be kept from causing power loss.
In the deposited film forming system of the present invention, the cooling means, which surrounds the dielectric material, can also cool the dielectric material heated by the heat coming from the discharge space, and hence the heat generated in the discharge space can be shut out and the power supply conductor can be prevented from being heated, so that the power supply conductor can be kept from causing power loss.